A sheet processing apparatus heats a sheet having a toner image to decolor the toner, and stacks the heated sheet in a sheet storage section. Upper and lower sheets that are stacked in the sheet storage section may stick to each other because the toner still remains on the heated sheets even after the heating process to decolor the toner. In addition, as the weight of the sheets stacked in the sheet storage section increases, the upper and lower sheets at the bottom of the stack are more likely to stick to each other due to the pressure from the increased weight. As a result, in order to prevent the sticking of sheets caused by the weight, it is necessary to urge a user to remove the sheets at an early time, i.e., before the stacked sheets stick to each other due to the pressure from the increased weight of the stack. In a case where the sheet removing time is set too early, the sheet removing operation performed by the user becomes more and more frequent, and this leads to lower productivity.